# fantasy-laws

2.0.1 • Public • Published

# fantasy-laws

Property-based tests to verify the lawfulness of Fantasy Land -compliant algebraic data types.

## Installation

Add `fantasy-laws`, `jsverify`, `sanctuary-show`, and `sanctuary-type-classes` to `"devDependencies"` in package.json, then run `npm install`.

## Usage

Usage is best explained by example. The following code defines a Sum type which is intended to satisfy Setoid, Semigroup, Monoid, and Group:

```function Sum(value) {
if (!(this instanceof Sum)) return new Sum (value);
this.value = value;
}

//  Sum.fantasy-land/empty :: () -> Sum
Sum['fantasy-land/empty'] = function() { return Sum (0); };

//  Sum#fantasy-land/equals :: Sum ~> Sum -> Boolean
Sum.prototype['fantasy-land/equals'] = function(other) {
return Z.equals (this.value, other.value);
};

//  Sum#fantasy-land/concat :: Sum ~> Sum -> Sum
Sum.prototype['fantasy-land/concat'] = function(other) {
return Sum (this.value + other.value);
};

//  Sum#fantasy-land/invert :: Sum ~> () -> Sum
Sum.prototype['fantasy-land/invert'] = function() {
return Sum (-this.value);
};```

The following steps demonstrate how to test the Group laws:

1. Import `fantasy-laws`, `jsverify`, `sanctuary-show`, and `sanctuary-type-classes`:

```import laws from 'fantasy-laws';
import jsc from 'jsverify';
import show from 'sanctuary-show';
import Z from 'sanctuary-type-classes';```
2. Import the type to be tested:

`import Sum from '../Sum.js';`
3. Define an "arbitrary" for the type:

```//    SumArb :: Arbitrary Sum
const SumArb = jsc.number.smap (Sum, sum => sum.value, show);```
4. Provide the fixed parameters to `laws.Group`:

`const {leftInverse, rightInverse} = laws.Group (Z.equals, Sum);`
5. Provide the appropriate number of arbitraries to the function associated with a particular law to produce a thunk:

```//    testLeftInverse :: () -> Undefined !
const testLeftInverse = leftInverse (SumArb);

//    testRightInverse :: () -> Undefined !
const testRightInverse = rightInverse (SumArb);```
6. To run the tests, invoke the thunk or use a test runner such as Mocha:

```suite ('Group laws', () => {
test ('left inverse', testLeftInverse);
test ('right inverse', testRightInverse);
});```